In order to improve the quality of formed glass products, there has been used a vacuum degassing apparatus which removes bubbles generated in molten glass before the molten glass that has been molten in a melting tank is formed by a forming apparatus, as shown in FIG. 3.
The vacuum degassing apparatus 110 shown in FIG. 3 is used in a process wherein molten glass G in a melting tank 120 is vacuum-degassed and is continuously supplied to a subsequent treatment vessel. In the vacuum degassing apparatus are provided a vacuum housing 112 which is evacuated to be depressurized therein, a vacuum degassing vessel 114 which is provided in the vacuum housing 112 and is depressurized together with the vacuum housing, and an uprising pipe 116 and a downfalling pipe 118 which are connected to respective ends of the vacuum degassing vessel in a downward and vertical direction. The uprising pipe 116 has a lower end immersed in the molten glass G in an upstream pit 122 in communication with the melting tank. Likewise, the downfalling pipe 118 has a lower end immersed in the molten glass G in a downstream pit 124 in communication with the subsequent treatment vessel (not shown).
The vacuum degassing vessel 114 is substantially horizontally housed in the vacuum housing 112 which is evacuated by a vacuum pump (not shown) to be depressurized therein. Since the inside of the vacuum degassing vessel 114 is depressurized to a pressure of 1/20-1/3 atmosphere together with the inside of the vacuum housing 112, the molten glass G in the upstream pit 112 before degassing is sucked and drawn up by the uprising pipe 116, and is introduced into the vacuum degassing vessel 114. After the molten glass has been vacuum-degassed in the vacuum degassing vessel 114, the molten glass is drawn down by the downfalling pipe 118 to be taken out into the downstream pit 124.
In order to evacuate the inside of the vacuum housing 112 through a suction port 112c by the vacuum pump or the like (not shown) to depressurize the inside of the vacuum degassing vessel 114 to a certain pressure and to maintain the depressurized state, the vacuum degassing vessel 114 has an upper portion formed with suction ports 114a, 114b to be open toward the inside of the vacuum housing 112.
Around the vacuum degassing vessel 114, the uprising pipe 116 and the downfalling pipe 118 in the vacuum housing 112 is provided thermal insulation material 130, such as refractory bricks, to cover these members for thermal insulation.
Since the conventional vacuum degassing apparatus 110 is constructed to deal with the molten glass G having a high temperature, such as a temperature at 1,200-1,4000.degree. C., paths for the molten glass in direct contact with the molten glass G, such as the vacuum degassing vessel 114, the uprising pipe 116 and the downfalling pipe 118, are constituted by circular shells which are made of noble metal, such as platinum and platinum alloy, as shown in JP-A-2221129.
The reason why the paths for molten glass, such as the vacuum degassing vessel 114, the uprising pipe 116 and the downfalling pipe 118, are made of noble metal, such as platinum and platinum alloy, is that there is no inclusion of impurities into the molten glass G and a certain strength is ensured at high temperatures since it is hardly possible due to low reactivity of the noble metal with the molten glass at a high temperature that, when the noble metal contacts the molten glass G at a high temperature, the noble metal elutes by reaction with the molten glass G.
When the paths for molten glass, such as the vacuum degassing vessel 114, the uprising pipe 116 and the downfalling pipe 118, are made of noble metal, such as platinum and platinum alloy, the following problems are created especially in building the vacuum degassing apparatus 110 in a large size:
(1) It is required that the temperature of the molten glass G at the inlet of the vacuum degassing apparatus 110 be not higher than 1,400.degree. C. PA0 (2) The cost greatly increases.
When the temperature is higher than 1,400.degree. C., the strength of the noble metal lowers. In order to set the temperature at a temperature not higher than 1,400.degree. C., the temperature in the melting vessel 120 can not be raised. This could lead to insufficient elution of glass material in the melting vessel 120.
In order to deal with a large quantity of molten glass, the sectional area of the paths is required to be large. This requires that the wall of the paths be thick to maintain the required strength for the paths, remarkably increasing the required quantity of the expensive noble metal and significantly raising the cost.
In terms of mainly cost reduction, a proposal has been made to constitute the paths for molten glass, such as the vacuum degassing vessel 1140, the uprising pipe 116 and the downfalling pipe 118, in the conventional vacuum degassing vessel 110 shown in FIG. 3 by refractory bricks more inexpensive than noble metal, such as platinum and platinum alloy (e.g., platinum-rhodium alloy), in order to build the apparatus in a large size and increase the degassing throughput of the molten glass.
However, there are limits to which refractory bricks are formed in a large size. It is absolutely impossible to build each of the vacuum degassing vessel 114, the uprising pipe 116 and the downfalling pipe 118 from a single refractory brick. In order to constitute the vacuum degassing vessel 114, the uprising pipe 116 and the downfalling pipe 118 of the vacuum degassing vessel 110 by refractory bricks, many refractory bricks are required to be combined. This means that joints are inevitably formed between the refractory bricks in the paths in direct contact with the molten glass.
Even if, in order to eliminate the provision of gaps at the joints of the refractory bricks, the vacuum degassing vessel, the uprising pipe and the downfalling pipe are carefully assembled using joint material and so on, thermal expansion of the refractory bricks easily creates gaps at the joints of the refractory bricks since the uprising pipe, the vacuum degassing vessel and the downfalling pipe of the vacuum degassing apparatus are heated until the temperature of the inner wall surfaces of these members reaches 1,200.degree. C.-1,400.degree. C. There is a possibility that the molten glass leaks through the gaps to shorten the life of the paths, and that the contact of the leaked molten glass with the thermal insulation material around the paths elutes components of the thermal insulation material to contaminate the molten glass in the paths, deteriorating the quality of glass products.